Witness accounts are famously unreliable [1] [2]. They are saying that the flight profile so far matches what they saw with Lion Air 610. Until the FDR and CVR are analyzed the only reliable data is what they have from ADSB and whatever Satcom they have.

[1] https://www.bostonglobe.com/metro/2015/05/23/eyewitness-acco...

[2] https://www.scientificamerican.com/article/do-the-eyes-have-...